Method for the removal of monomers, etc. from molten polymers



Jan. 17, 1956 T. s. MAYNER 2,731,081 METHOD FOR THE REMOVAL OF MONOMERS, ETC. FROM MOLTEN POLYMERS Filed April 15, 1953 CONDENSER INVENTOI? THOMAS 5. MA YNER A TTOR/VEY 2,731,081 1C Patented Jan. 17, 1956 METHOD FOR THE REMOVAL OF MONOMERS, ETC. FROM MOLTEN POLYMERS Thomas S. Mayner, Chagrin Falls, Ohio, assignor to Industrial Rayon Corporation, Cleveland, Ohio, :1 corporation of Delaware Application April 13, 1953, Serial No. 348,503

5 Claims. (Cl. 159-49) This invention relates to the production of shaped articles such as filaments, monofils, bristles, films and the like from Nylon 6 type polyamide polymers. More particularly, it relates to a process of melt spinning polyamides produced by the polymerization of lactam monomers, e. g., epsilon-amino caprolactam hereinafter referred to as e-caprolactam.

In the polymerization of e-caprolactam it is known that the polymerization reaction does not proced to completion and that at the end of the polymerization reaction a state of equilibrium exists between the polymer and the monomeric or dimeric lactam wherein the polymerization product contains up to about 10% of the unpolymerized lactams.

It is also known that the presence of such lactams in the extruded shaped articles, hereinafter referred to as yarn, present spinning and processing difiiculties. Thus, it may be necessary to interrupt a continuous process operating at increased rates with slow and expensive washing steps prior to or after extrusion. Also, there is a loss of yarn strength caused by the handling of such lactam containing yarns, e. g., during the transfer of such yarns from bobbins, over guides or during stretching and rewinding operations.

Heretofore there have been a number of proposals for meeting the problems created by the presence of excessive lactam in either the spinning melt or in the extruded product. In one proposal the polyamide melt containing excessive lactams is first converted into solid chips, ribbons or beads and the latter washed with warm water to extract the water-soluble lactam and the resulting washed polymer thereafter subjected to an expensive drying operation. In addition'to the disadvantages arising out of the expense of this proposal, it is found that monomeric lactam is reformed when the extracted chips are again melted and maintained at the melt temperature prior to extrusion. Thus, a depolymerization reaction takes place in which up to 5% or more of the lactam is reformed depending upon the length of time it is maintained in the fluid melted state.

In another proposal the polymer containing excessive lactam is melt-spun and the resulting solidified yarn is coated with a lubricant so as to prevent filament breakage as the yarn passes over handling means such as guides, spools, etc. during the subsequent processing steps, e. g., stretching, packaging, unwinding, etc. More specifically, the filament breakage referred to is caused by the accumulation of the lactam (a white solid having a melting point of 68 C.) on the handling or guiding means as the yarn passes thereover. Accordingly, the lubricant is added to reduce the amount of hard deposits formed on the guides or other associated yarn contacting devices. This proposal, however, does not provide a satisfactory solution to the problem. The yarn damage caused by the lactam deposits is not sufiiciently eliminated and, in addition, the presence of the lubricant on the yarn renders it more difficult and expensive to leach out the lactam from the fiber. Moreover, the presence of the lubricant increases both the cost of the process and the cost of recovering the lactam from the wash waters.

In accordance with the present invention, a melt spinning process has been discovered which not only avoids the necessity of forming and washing polymer solid chips, etc., prior to extrusion, but also successfully avoids the loss of yarn strength because of excessive or undesirable quantities of lactam. The process of the present invention may be performed as follows. The fluid melt of the polymer containing excessive or undesirable proportions of the lactam monomers, dimers, etc., is converted prior to spinning into the form of a continuous spreading flow in an enclosed vessel; the flow being advantageously spread by a frusto-conical surface or surfaces. The melt is initially admitted into a circular trough having base openings onto either side of a frusto-cone or cones positioned directly below. The melt thus is provided with a large area or surface over which it continuously flows as a spreading thin film. The excess or undesirable quantities of lactam monomers, etc. that rise, and that are released during the flow and that evaporate from the thinned exposed melt surface are continually removed or exhausted from the chamber. The flow of polymer, meanwhile is continuously subjected to a sparging action of an inert gas which can be superheated steam or nitrogen gas, or carbon dioxide which assists greatly in evaporating and in removing rising lactam monomers, etc., and any vapor that is given off, and this mixture of bases being continually evacuated. The exhaust mixture is subsequently condensed in a zone removed from said melt thereby removing the extracted monomers from the melt.

The process is further more specifically illustrated and described in the following specification and accompanying drawing, where:

The drawing is a schematic arrangement of an apparatus which is advantageously adapted for de-monomerizing a polyamide melt.

Referring to the drawing a source of polymer melt 10 continually supplies melt to an enclosure or chamber 12 through conduit 11. The conduit 11 feeds enclosed trough 14 with a polymer melt. The melt then flows from the trough through openings 15, 16, onto the inner and outer surfaces of frusto-cones 18, 19. The frusto-cones are concentrically positioned in the enclosure 12 and relative each other. The melt is spread by gravity flow finally dropping as a circular sheet or cylinder into the converging conical base 20 of the enclosure 12 from whence it is forced into an extruding device 22 by the pump 21, both devices being only generally shown. As the melt flows downwardly into the base 20 it is being continually swept in counterflow by an inert gas. The gas is advantageously utilized as a de-monomerizing assistant.

The inert gas is supplied by conduit 24 under some pressure and spread by a circular sparger 25 also concentrically positioned within the inner frusto-cone 19. The gas sweeps the inner surfaces of each frusto-cone as well as the outer surfaces, rising through openings 23 in the circular trough 14 into the upper section 30 which is exhausted by conduit 31 being joined with a condenser 32.

The temperature of the inert gas and of the melt are closely controlled within desirable ranges for effecting the greatest release of monomers and of vapor. The entire apparatus is lagged for a close temperature control. The operation herein recited can be under atmospheric pressures but it is more desirable that the enclosure 12 be under vacuum for more eifective release of undesirable quantities of monomers, and vapors. The inert gas should be at a temperature equal to and preferably somewhat greater than the temperature of the polyamide melt depending upon the degree of polymerization and the proportion of lactams in the melt. The melting temperature of the polymer may be between 210 C. and 220 C. However, to accelerate the rate of removal it is found advantageous to maintain the melt at temperatures substantially above the melting point, e. g., between 230 C. and 260 C. and accordingly, employing similar or greater temperatures for the inert gas. Moreover, it has been found that even greater advantages are derived when the inert gas is employed as a current or stream having an appreciable velocity impinging in a scrubbing or washing action on the flowing thin film of the melt.

The duration of treatment of the flow under the above conditions may be varied depending upon the amount of lactam desired in the spun product. Thus, proportions of lactam from 3 up to 5 or 6% by weight not only may be tolerated but may be advantageous in their afiect on spinning and the character of the product. It is prefered, however, to reduce the lactam content to below about 4%.

I claim:

1. A process for continuously evaporating undesirable quantities of monomers from a melt of polyamides obrained by the polymerization of a material selected from the group consisting of e-caprolactam and e-amino caproic acid which comprises, in a heated enclosure continuously gravitationally spreading a melt of said polyamides over the inner and outer surfaces of a right circular frusto-cone, allowing said melt to drop freely into a collecting sump from the lower edge of the frusto-cone, meanwhile subjecting said spreading melt to a sweeping fiow of an inert gas; no additional heat being supplied to the flowing melt other than that contained in the melt and in the said gas, and continuously withdrawing the volatilized monomers and said inert gas.

2. A process as described in claim 1 in which the inert gas is superheated, steam.

3. A process as described in claim 1 in which the inert gas is nitrogen.

4. A process for continuously evaporating undesirable quantities of monomers from a melt of polyarnides obtained by the polymerization of a material selected from the group consisting of e-caprolactam and e-amino caproic acid which comprises, in a heated enclosure continuously gravitationally spreading a melt of said polyarnides over the inner and outer surfaces of each of a plurality of right circular frusto-cones, allowing said melt to drop freely into a collecting sump from the lower edges of the frusto-cones, meanwhile subjecting said spreading melt to a sweeping flow of an inert gas; no additional heat being supplied to the flowing melt other than that contained in the melt and in the said gas, and continuously withdrawing the volatilized monomers and said inert gas.

5. A process as described in claim 4 in which. said plurality of expanding surfaces are concentric.

References Cited in the file of this patent UNITED STATES PATENTS 2,238,204 Woods Apr. 15, 1941 2,489,768 Firth Nov. 29, 1949 2,640,761 Wiseman June 2, 1953 FOREIGN PATENTS 144,631 Great Britain May 5, 1921 497,507 Belgium Dec. 1, 1950 

1. A PROCESS FOR CONTINUOUSLY EVAPORATING UNDESIRABLE QUANTITIES OF MONOMERS FROM A MELT OF POLYAMIDES OBTAINED BY THE POLYMERIZATION OF A MATERIAL SELECTED FROM THE GROUP CONSISTING OF E-CAPROLACTAM AND E-AMINO CAPROIC ACID WHICH COMPRISES, IN A HEATED ENCLOSURE CONTINUOUSLY GRAVITATIONALLY SPREADING A MELT OF SAID POLYAMIDES OVER THE INNER AND OUTER SURFACES OF A RIGHT CIRCULAR FRUSTO-CONE, ALLOWING SAID MELT TO DROP FREELY INTO A COLLECTING SUMP FROM THE LOWER EDGE OF THE FRUSTO-CONE, MEANWHILE SUBJECTING SAID SPREADING MELT TO A SWEEPING FLOW OF AN INERT GAS; NO ADDITIONAL HEAT BEING SUPPLIED TO THE FLOWING MELT OTHER THAN THAT CONTAINED IN THE MELT AND IN THE SAID GAS, AND CONTINUOUSLY WITHDRAWING THE VOLATILIZED MONOMERS AND SAID INERT GAS. 